Contact Binaries

Some stars, known as contact binaries, are so close together that they actually touch and exchange the gas in their outer layers. Although the two stars may be quite different, as in this illustration of the system known as W Ursae Majoris, this exchange gives both stars the same surface temperature and appearance. Eventually, the two stars may merge to form a single star.

Many stars have close companions — other stars that are so close that the two whirl around each other once every few days. But a few take closeness to the extreme — they enfold each other in a warm embrace.

The first “contact binary” ever discovered is known as W Ursae Majoris. It’s in Ursa Major, the great bear, which is the home of the Big Dipper. The Dipper is in the northwest at nightfall, with faint W Ursae Majoris well to its right.

More than a century ago, two German astronomers noticed that the system’s brightness varied by a factor of two every four hours. They considered several possibilities for the rapid change. Their best guess was that the two stars were so close together that they eclipsed each other every four hours.

Later observations found that one star is a bit bigger and heavier than the Sun, while the other is smaller. But the two stars are the same brightness and color — something that shouldn’t happen for two stars of different mass. That means the stars are not just touching each other, but they’re also sharing the gas in their outer layers. The gas from the bigger star pumps up the temperature and brightness of the other.

The stars may have formed a bit farther apart and slowly spiraled together, or they could be the product of a single star that split apart soon after its birth. Either way, the two are among the closest of stellar siblings — locked in a perpetual embrace.

More about contact binaries tomorrow.

Script by Damond Benningfield, Copyright 2012

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